1. Field of the Invention
This invention relates to a process for the preparation of polyurethane elastomers and is more particularly concerned with improved catalysis for the preparation of polyurethane elastomers.
2. Description of the Prior Art
Thermoplastic non-cellular polyurethane elastomers are currently available for use in the preparation of articles by injection molding, extrusion and like techniques. These elastomers are generally prepared by the reaction of an organic diisocyanate, a polymeric diol, and a difunctional extender in the presence of a catalyst. Monomer purity is particularly critical both in terms of the effect it can have on the progress of the reaction by way of its effect on the catalyst and the effect it may have on the resultant polymer properties.
Polymeric diols including the types of polyester and polyether diols used in the preparation of polyurethanes are subject to peroxidic impurity formation. Such peroxidic impurities are not limited to polymeric diols but form also in low molecular weight diols of the type used for extending the polyurethane reaction. Peroxide content of diols used in polyurethane elastomer formation must be kept below certain critical levels otherwise unacceptable polymer properties or reactions result, if, indeed, the polyurethane can be prepared at all. Ideally, elastomer grade polymeric diols and diol extenders should not have peroxide contents above 10 p.p.m. (see Siggia. S., Quantitative Organic Analysis via Functional Groups, John Wiley and Sons, Inc., New York, N.Y., 1963, p. 255 for analytical method for determining peroxide content) and the prior art discloses the addition of various antioxidants to the diols and extenders to inhibit the formation of peroxides. However, diols and extenders not containing stabilizers or antioxidants, after storage or heating or the like, can easily have peroxide contents ranging up to 200-300 p.p.m. rendering them literally useless for the preparation of polyurethane elastomers. Heretofore, no method has been disclosed in which such contaminated materials can be used efficiently in the preparation of high grade elastomer products.
Thus, both the ingredients and the elastomers produced therefrom have been stabilized using various types of antioxidant and scavenging type additives. For example U.S. Pat. No. 2,915,496 discloses a method for preparing a heat and weather resistant polyurethane elastomer by the addition of an antioxidant material to the urethane forming ingredients. U.S. Pat. No. 3,205,269 discloses the stabilization of polyethers and polyether alcohols by the addition of certain phosphites, thiophosphites, cyclic phosphites and the like. The stabilized materials are used, typically, in the manufacture of polyurethanes. Stabilization of the diisocyanate component is also well known; see for example U.S. Pat. No. 2,950,307 and U.S. Pat. No. 3,715,381. Further, certain phosphite and phosphine compounds have been observed to catalyze the formation of integral-skinned polyurethane foams; see U.S. Pat. No. 4,021,381.
Phosphorus containing antioxidants, particularly the phosphites (see U.S. Pat. No. 3,205,269 cited supra), while being recognized as effective antioxidants, are generally avoided because of their tendency to hydrolyze in the presence of trace amounts of moisture. The phosphorus acids thereby produced over an extended period of time are extremely detrimental to the polyurethane forming reaction.
It is an object of the present invention to provide a method for the preparation of polyurethane elastomers which method can employ polymeric diols or diol extenders which contain higher than normally acceptable levels of peroxide impurity and which heretofore would not be considered useful for the preparation of high-grade polyurethane elastomers.
It is a further object to effect the polyurethane reaction in the presence of lower levels of the expensive polyurethane forming catalysts.
These objectives and others are readily accomplished by carrying out the polyurethane reaction in the presence of a particular stannous tin catalyst and using a polymeric diol and diol extender one or both of which have been treated with a particular phosphorus compound prior to the one-shot polyurethane formation.